1. Field of the Invention
The present invention relates to a liquid crystal display device, in particular to, an active matrix type liquid crystal display device. In addition, the present invention relates to a liquid crystal display device that can display gradation.
2. Description of the Related Art
The present invention relates to a liquid crystal display device, in particular to, a liquid crystal display device of less power consumption type. In addition, the present invention relates to a liquid crystal display device that can display gradation.
Since a liquid crystal display is thin and the power consumption thereof is small, it has been widely used for note type personal computers. In particular, an excellent feature of the liquid crystal display device over other types of display devices such as CRT and plasma display is low power consumption. The liquid crystal display device is expected to be used for portable information units.
In the case of a portable unit, the power consumption of the display thereof is preferably 500 mW or less, more preferably several mW or less. For such a requirement, so far, a reflection type liquid crystal display device of simple matrix and small power consumption type free of a back light with a TN (Twisted Nematic) liquid crystal has been used. However, since the TN type liquid crystal requires a polarized plate, the reflectance is around as low as around 30%. In addition, when the number of pixels of the simple matrix type liquid crystal display is increased, the contrast decreases and thereby the display picture quality deteriorates. To solve such a problem, a PCGH (Phase Change Guest Host type) mode liquid crystal that does not need a polarized plate is used. Moreover, with an active matrix, a display device with high reflection rate and high contrast has been developed.
FIG. 10 is a schematic diagram showing a circuit diagram showing the structure of a pixel of such a conventional liquid crystal display device. In the following, unless required, only one pixel will be described for simplicity. The structure of the circuit of a pixel shown in FIG. 10 is the same as the structure of a conventional transmission active matrix type liquid crystal display device. When a thin film transistor 91 is turned on corresponding to a scanning signal supplied to a gate line 94, the voltage of a data signal supplied to a signal line 95 is supplied to a liquid crystal layer 93. In addition, an electric charge is supplied to an auxiliary capacitor 92 through an auxiliary capacitor line (Cs) line 97. As well known, an AC voltage should be supplied to the liquid crystal layer 93. A voltage of a data signal that varies based on a voltage of an opposite electrode 96 formed on an opposite substrate is supplied to the signal line 95 so as to drive the pixel.
In such a liquid crystal display device, even if a picture displayed on the display does not change at all, it is necessary to supply an AC voltage to the liquid crystal layer. Thus, whenever the pixel is selected at a frame interval, the pixel voltage is rewritten. Since the power consumption P of which an AC voltage is supplied to the capacitor is expressed by the following formula. EQU p=f.times.V.sup.2 .times.C
where f=frequency; V=voltage; and C=capacitance.
Thus, the power consumption is proportional to each of the frequency, voltage, and capacitance.
When a liquid crystal display device is driven with an AC voltage, the drive frequency of each pixel is represented with a frame frequency. The drive frequency of the signal line is represented with the product of the frame frequency and the number of scanning lines. The drive frequency of a signal line driver IC is represented with the product of the number of pixels of the display and the frame frequency. When the liquid crystal display is separately driven, the drive frequency of the signal line driver IC is represented with the quotient of which the product of the number of pixels of the display and the frame frequency is divided by the number of separated regions. For example, in the case of a VGA type liquid crystal display device composed of 640.times.480 pixels (RGB), assuming that the frame frequency is 60 Hz and that each of RGB uses respective shift registers, the clock frequency of the signal line driver IC becomes 60.times.480.times.640 =18 MHz. Although the power consumption of the liquid crystal display device partly depends on the driving IC, the power consumption becomes around 200 mW. The frequency of each signal line becomes 60.times.480=29 kHz. Assuming that the diagonal length of the liquid crystal display device is 10.4 inches, the capacitance of each signal line is around 40 pF. When the display panel of the liquid crystal display device is driven, the power consumption becomes around 50 mW. When the number of pixels is increased (for example the display panel is composed of 1600.times.1200 pixels), since the power consumption thereof is proportional to the number of gate lines, the power consumption of this display panel becomes 2.5 times as large as that of the conventional display panel composed of 640.times.480 pixels. In addition, since the power consumption of the driver IC is increased with the similar rate, the total power consumption of the apparatus increases to around 1 W. When a portable information unit has a liquid crystal display device with a large power consumption, the battery of the unit runs out in a short time. Thus, the operation time of the unit becomes short. To prolong the operation time of the unit, a large (heavy) battery should be used.
To reduce the power consumption, a surface stabilized ferroelectric liquid crystal (SSFLC) can be used. The SSFLC has a memory characteristic. Thus, unless a picture displayed on the display is changed, the voltage supply can be stopped. However, in the SSFLC, the orientation of the liquid crystal becomes disordered with a shock and thereby a picture is not correctly displayed. Thus, the SSFLC cannot be used for a portable display device. In addition, a liquid crystal with a memory characteristic occasionally has restrictions with respect to contrast and reflection rate. Thus, the display quality of such a liquid crystal has a problem of the display quality. For example, the SSFLC requires a polarizing plate. In addition, since the reflection rate of the SSFLC is as low as around 30%, a picture on the display becomes the brightness decreases. Moreover, due to the characteristic of the SSFLC, since a picture is basically displayed in binary display mode rather than gradation display mode, the display characteristic (information amount) of the SSFLC is much lower than that of the apparatus with the gradation display mode. This is a notable drawback of the SSFLC when it displays color pictures. To display gradation, if a space modulation corresponding to for example dither method is used, the effective resolution is deteriorated. When a time modulation corresponding to frame rate control method is performed, a picture on the display flickers. Thus, the SSFLC cannot be used for moving pictures.
As described above, personal computers and portable information units mostly deal with still pictures. Thus, even if a picture is not changed, an AC voltage is supplied to a signal line. Thus, the power is wasted.
An object of the present invention is to solve the above-described problem and to provide a liquid crystal display device of less power consumption type.
Another object of the present invention is to provide a liquid crystal display device for allowing a gradation signal to be supplied to a liquid crystal so as to display data greater than binary data.